Ordered to lie flat to ease a back condition, the American talkshow host Ellen DeGeneres is currently presenting her programme from a bed on the studio floor. While Alistair Cooke once recorded Letter From America in hospital, listeners were unaware of his position and so DeGeneres's decision, which unexpectedly merges the grammar of daytime television with that of ER, is something of a broadcasting first, although various presenters in the past have used footage of their colonoscopies or other procedures in their shows.

DeGeneres is an actor by profession and so her reluctance to take time off may be encouraged by that oldest of theatrical mottos, that the show must go on. While often pilloried as self-pitying indulgence, this belligerent attitude to illness has practical roots in the fact that performers tend to be freelances, who only get paid if they check in, and that few companies can afford understudies, although on the more lavish musicals it's said that there have been nights during the flu season when every leading cast member and most of the orchestra have sent in the cover.

But, perhaps, on some such evenings the audience were better served by the absence of the advertised players because the desire to keep the show going can almost stop it. In a London production of Stephen Sondheim's Follies, Dolores Gray, having damaged her foot, was wheeled on stage for her big torch song, mounted on a little platform attached to the piano. In another Sondheim, Sweeney Todd at the Royal Opera House, a singer playing one of the barber's victims had been hurt in a motorbike accident and so had to dramatise his throat-slit death by limping off stage rather than being tipped backwards out of Sweeney's chair.

Yet, though sometimes adding unwanted comedy to productions, resistance to sick leave can also create extraordinary tragic force. The fine National Theatre actor Michael Bryant, whose final role in Chekhov's The Cherry Orchard involved dying every night on stage, was, his family and friends knew, mortally ill as he pretended his end. The gravely sick Ian Charleson, also at the National, was a Hamlet who delivered "To be or not to be" in the knowledge of his own imminent extinction. The remarkable power of Jason Robards's performance as a dying man in Magnolia was poignantly explained by his own death from cancer shortly afterwards. There is a great nobility in this, yoking even infirmity to your talent in a way that most professions simply can't.

But the theatrical convention of carrying on despite handicap also applies to that other branch of showbusiness: democracy. This week's preview extracts from Ronald Reagan's soon to be published diaries ("Getting shot hurts") are a reminder of the moment when, with an actor as American president, the greasepaint rules of not letting the pain show were most openly applied in politics.

After the 1981 assassination attempt, the immediate release of pre-operative quips ("Gee, hon, I forgot to duck"), and rapid scheduling of a recovery room photo-opportunity, reflected a belief that the show must be seen to go on which rivalled even that of the most determined trooper crawling up the stagedoor stairs towards the wings with the emergency room intravenous drip trailing from their arm.

Play-acting about the health of national leaders is, though, understandable, because of the perception that sections of the audience may take pleasure in weakness. This was a time when Soviet missiles were aimed at Washington and, even with the cold war over, currencies may crash or terrorists take heart if the guy on the end of the hotline looks or sounds like he's going to croak. The increasingly widespread tendency for male politicians to wear makeup in daylight - started by Reagan but industrialised by Blair - confirms this theatricalisation of politics.

However, a far more questionable application of the maxim to high office was the recent decision of John Edwards to continue his presidential bid despite his wife's diagnosis with late-stage breast cancer. Just as the thespian tradition of continuation has sometimes resulted in unjustifiable physical risk, so it can seem absurd that with family time limited, the vote-getting show should go on in these circumstances.

There is, though, a significant difference between the Edwardses and previous examples of political troopers. Reagan's recovery from being shot was, like the earlier conspiracy to prevent the public from seeing Franklin Roosevelt's wheelchair, an attempt at disguising how bad things really were. John and Elizabeth Edwards haven't done that but, rather like some of the actors discussed above, have incorporated their pain into their show, although, unlike performers who are operating the double bluff of pretending to be suffering what they really are, they run the risk of being accused of seeking sympathy from viewers.

However, while a show that goes on in distressing circumstances needs to be a show that's worth doing, rising levels of sick leave and absenteeism suggest that an attitude which can be dismissed as a luvvie reflex might usefully be more widely applied.